How do I change strength of emission at Principled BSDF
Blender 2.92 + :
In the newer versions the principled BSDF shader actually has a field for emission strength, right below the emission color field. You can use it much like the simpler Emission Shader.
You can still input color values superior to 1, but there is no need anymore with the new strength field.

Older Versions :
You can manually input values higher than 1 in the red,green,blue fields of the emission color of the principled shader. Keep in mind that this is a workaround and will not mimic exactly how a classic emission shader works when you tweak its strength.

For ease of use you can use such a setup : Add an rgb node in order to input the exact color you want the emission to be, a value node for the strength of the emission, and link both of them to the color inputs of a mix rgb node set to multiply with a factor of 1, then set the output of the mix rgb to the emission input of the bsdf.